1. Field of the Invention
The present invention relates to a DC/DC converter and an audio device incorporating the same, and, more specifically, relates to a DC/DC converter used in a power source circuit which produces power source voltage by boosting a voltage corresponding to a single dry battery, or the like, for audio devices driven by a single dry battery, or the like, which are such as a portable radio recever, a portable CD player and a portable magnetic tape player. The present invention also relates to a DC/DC converter which is easily manufactured and further permits reduction in power consumption.
2. Background Art
The portable radio receiver, the portable CD player and the portable magnetic tape player are usually driven by a dry battery and the number of dry batteries used therefor is mostly one or two. Therefore, the power source voltage therein is limited up to only about 1.2 V or about 2.4 V. Moreover, an audio device, such as a portable CD player and portable magnetic tape player, incorporates a motor therein and further includes a control circuit constituted by a microcontroller (MCU) or a microcomputer (MPU) and a ROM and the like which operates many kinds of circuits in response to many kinds of operating signals.
The operating voltage of the control circuit constituted by the microcontroller or the microcomputer and the ROM and the like is usually at about 3 V.about.5 V. Further, an audio signal processing circuit is usually driven by a power source voltage of about 2.4 V.about.3.5 V. In particular, the motor driving circuit for these types of devices requires a higher voltage equal to or more than about 3.5 V, because such a circuit uses a MOS transistor in order to reduce the power consumption therein. For this reason, the power source circuit used in these types of devices is provided with a DC/DC converter which produces a higher predetermined power source voltage by boosting a voltage of about 1.5 V.
Now, the audio devices such as portable CD players and portable magnetic tape players are required to be driven for a long time, and moreover are desired to be driven only by a single dry battery. In order to realize the long time drive it is important to reduce the power consumption in their inner circuits, and until now circuits which are driven at a low voltage and with a low power have been developed and used for the respective devices.
Further, as one of the countermeasures for reducing the power consumption in the power source circuit for these types of devices, a great importance is placed on the reduction of power consumption during a no load period wherein even when the device is in an operable condition substantially no current flows through the load. In order to suppress power consumption during the no load period, major circuits such as a DC/DC converter in the power source circuit are usually constituted by CMOS circuits. With this measure the power consumption during the no load period can be reduced in comparison with a DC/DC converter constituted by bipolar transistors, because the driving current caused flowing through the gate electrode of a MOS transistor is far less than that caused flowing through the base of a bipolar transistor.
However, except for CMOS transistors which are manufactured by a special transistor manufacturing process effective for reducing the operating voltage of the transistors, the operating threshold voltage of CMOS transistors in a DC/DC converter constituted by a usual CMOS circuit is higher than that of a bipolar transistor. For this reson it is difficult to operate a DC/DC converter constituted by such a CMOS circuit with the voltage determined by a single dry battery or the like.
One of DC/DC converters for these types of devices which is also designed to reduce the power consumption is disclosed in a copending U.S. patent application Ser. No. 08/449,830 field on May 24, 1995 now U.S. Pat. No. 5,565,762, which is incorporated herein by reference.